5. Vibration Measurement and Control

Measurement and Control of Vibration | THEORY

UNIT 5: Measurement and Control of Vibration (8 Hrs)

Contents:
A) Measurement: Vibration Measuring Instruments, Accelerometers, Impact hammer,

Vibration shakers, Vibration Analyzer, Vibration based condition monitoring,
Analysis of Vibration Spectrum, Standards related to measurement of vibration,
Human response to vibrations.

B) Control: Vibration control methods, passive, semi active (Introduction to Electro-
Rheological & Magneto-Rheological dampers) and active vibration control, control
of excitation at the source, control of natural frequency, Vibration isolators, Tuned
Dynamic Vibration Absorbers, Introduction to Torsional Damper

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Measurement and Control of Vibration | THEORY

A) VIBRATION MEASUREMENT:

1.1 WHY WE NEED TO MEASURE VIBRATIONS?
 To measure natural frequencies for selecting operational speed to avoid resonance.
 To verify theoretical values, it may be different from measured values.
 To design active vibration isolation systems.
 To identify mass, stiffness and damping of a system.
 To detect shifts in natural frequency which indicates possible failure.
 To verify the approximated model.

1.2 BASIC STEP OF VIBRATION MEASUREMENT

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Measurement and Control of Vibration | THEORY

1.3 CLASSIFICATION OF VIBRATION MEASURING INSTRUMENTS
1. Classification base on contact

1. Contact type
2. Non-contact type
2. Classification base on display method
1. Indicating type
2. Recording type
3. Classification base on time base measurement
1. Real time based
2. Non-real time based
4. Classification base on power source
1. Active system
2. Passive system

1. CLASSIFICATION BASED ON CONTACT
I. Contact type These types of vibration measuring instruments are in direct contact with
the vibration machines. These instruments are compact in size. e.g. Accelerometer
II. Non-contact type These types of vibration measuring instruments are used when it is
very difficult to use the contact type vibration measuring instruments. These types of
instruments also small in size.

2. CLASSIFICATION BASED ON DISPLAY METHOD
I. Indicating type in these instruments, the measured data are displayed on the display
unit of the instruments.
II. Recording type These instruments are used to display and also to record the data for
future analysis. e.g. FFT analyzer

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Measurement and Control of Vibration | THEORY

3. CLASSIFICATION BASED ON TIME BASE MEASUREMENT
I. Real time based The real time based data can be measured using these instruments.
These instruments are working based on microprocessor.

II. Non-real time based These instruments are not real time based. The measure data can
only display on the display unit of the instruments.

4. CLASSIFICATION BASED ON POWER SOURCE
I. Active system in these instruments, source of power is required to operate the
instruments for vibration measurement. e.g. FFT analyzer
II. Passive system These instruments do not require any outside source of power to
operate the instruments. They are compact, handy and battery operated. e.g. Frahm’s
tachometer

1.4 VIBRATION MEASURING INSTRUMENTS
A) Displacement measuring instrument (Vibrometer)
B) Velocity measuring instrument (Velometer)
C) Acceleration measuring instrument (Accelerometer)
D) Frequency Measuring Instruments

A) Displacement measuring instrument (Vibrometer)
 A vibrometer or a seismometer is an instrument that measures the displacement of

a vibrating body.
 It consists of a mass-spring damper system mounted on the vibrating body.
 The relative displacement between the mass and the base sensed by the

transducer.  Relative displacement analysis by this equation.

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Measurement and Control of Vibration | THEORY

 Vibrometer or Seismometer is an instrument which measures the displacement of a
vibrating machine or structure.  It can be classified into following types:
1. Stylus Recording Instrument
2. Seismic Instrument or Seismometer or Vibration Pickup
3. Optical Recording Instrument
4. Simple Potentiometer
5. Capacitance Pickup
6. Mutual Inductance Pickup

B) Velocity measuring instrument (Velometer)
 A velometer measures the velocity of a vibrating body.
 Frequency ratio must be very large.
 Velocity of vibrating body computed by

C) ACCELEROMETER
 Accelerometer used to measure the acceleration of vibrating body.
 It is widely used to vibration measurement and also to record earthquakes.
 From the accelerometer record, the velocity and displacements are obtained by

integration.
 Two type of accelerometer are commonly used compression type and shear type.
 Mostly piezoelectric transducer used in accelerometer

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Measurement and Control of Vibration | THEORY

 ADVANTAGES OF ACCELEROMETER
 Frequency ratio is small so, instrument required large natural frequency.
 It need to small mass and high spring stiffness, so the instrument will be small size.
 It exhibits better all-round characteristics than any other type of vibration

transducer.
 It has very wide frequency and dynamic ranges with good linearity throughout the

ranges.
 Additionally, the piezoelectric accelerometer is self-generating, so that it doesn't

need a power supply

1.5 FREQUENCY MEASURING INSTRUMENTS

 This instrument is based on principal of resonance.
Types:

 1. Single reed frequency meter or fullatron tachometer
 2. Multi reed frequency meter or frahm tachometer
 3. Stroboscope

1.6 SIGNAL CONVERSION INSTRUMENT

 In signal conversion capture vibration signal from vibration pickup device and
present it in convenient form.

 Signal conversion done by two step.
A) Signal conditioning
B) Signal analyzer

A) SIGNAL CONDITIONING
 A signal conditioner is a device that converts one type of electronic signal into an

another type of signal.
 Its primary use is to convert a signal that may be difficult to read by conventional

instrumentation into a more easily read format.
 In performing this conversion, a number of functions may take place.

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Measurement and Control of Vibration | THEORY

B) SIGNAL ANALYZER

 These devices analyze a signal in the frequency domain by separating the energy
of the signal into various frequency bands.

 The separation of signal energy into frequency bands is accomplished through a
set of filters.

 The analyzers are usually classified according to the type of filter employed.
 In recent years, digital analyzers have become quite popular for real-time signal

analysis.
 There are two types of real-time analysis procedures the digital filtering method

and the fast Fourier transform (FFT) method

1.7 FFT SPECTRUM ANALYZER

 Fourier transform is a mathematical procedure to obtain the spectrum of a given
input signal.

 A mathematical set of data points can be converted to a spectrum using Fourier
transformation program in digital computer.

 Thus the method to obtain the spectrum using computer is called as fast Fourier
transform. (FFT).

 The instrument which convert the input signal with time as a independence variable,
into frequency spectrum and displays it in graphical form is called as spectrum
analyzer or FFT Analyzer

 TYPES OF FFT ANALYZER
A) Hand Held FFT Analyzer
B) Computer Based FFT Analyzer
C) Single Channel FFT Analyzer
D) Two Channel FFT Analyzer
E) Multi-Channel FFT Analyzer

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Measurement and Control of Vibration | THEORY
A) HAND HELD FFT ANALYZER
 Both measurement and analysis is done in same instrument.
 Data can be collected from actual field for analysis.

B) COMPUTER BASED FFT ANALYZER
 PC Based analyzer is commonly used,
 it is very compact and portable.
 Only vibration measurement is done the analysis can be done with the help of

software.

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Measurement and Control of Vibration | THEORY

C) SINGLE CHANNEL FFT ANALYZER
 Suitable for continuous and transient vibration measurement in environmental and

industrial applications.
 Single data can be measure at a time i.e. accelerometer or microphone can be

attached to measure vibration or microphone.

D) TWO CHANNEL FFT ANALYZER
 Simultaneously 2 input can be taken or connected i.e. accelerometer and

microphone can be connected at the same time.
 Or Same type of data can be measure for different points.

E) MULTI CHANNEL FFT ANALYZER
 When it is necessary to measure the data of more than two point on a machine then

this is used.
 This is multipurpose and can be used in combination with all relevant vibration and

sound transducers.

 APPLICATIONS OF FFT ANALYZER
 Used for obtaining the frequency response characteristics of vibrating structure or

body.
 Used in experimental modal analysis or model testing for determination of natural

frequency, damping ratio, mode shape etc.
 Used in vibration and noise monitoring system.

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Measurement and Control of Vibration | THEORY

1.8 VIBRATION SHAKER OR EXCITERS

o A vibration exciter is a machine which produces mechanical vibratory motion to test
object.

o It is designed to produce a given range of harmonic or time dependent excitation
force and displacement through a given range of frequencies.

 TYPES OF EXCITERS
o The three major types of vibration exciters are commonly used in several

applications as:
A) Mechanical exciters

B) Electro-dynamic exciter

C) Electro-hydraulic exciter

A) MECHANICAL EXCITERS
o Mechanical exciters which vibrate the structure or machine by mechanical system.
o Types of Mechanical Exciters: According to applied force mechanical exciters are

three types.
1.Inertia force mechanical exciters
2.Elastic spring force mechanical exciters
3.Unbalanced force mechanical exciters

 ADVANTAGES OF MECHANICAL EXCITERS
o The generated forces are transmitted directly to the table without dependence

upon a reactionary force against a heavy base or rigid ground connection.
o The cost of mechanical exciter is less compared to other exciters.
o There are no leakage problems as in hydraulic exciters.
o Defective electrical components and connections fail under the induced vibration.
o This exciter eliminates embarrassing and costly repair and difficult tracing of circuits

in the field.

 DISADVANTAGES OF MECHANICAL EXCITERS
o Mechanical exciter cannot be used in high temperature, humidity and altitude

environments.
o It can only be used for small applications.
o The frequency range is small compared to hydraulic and pneumatic

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Measurement and Control of Vibration | THEORY
B) ELECTRO-DYNAMIC-EXCITER
 Electro-dynamic force applied to vibrate the structure.
 The magnitude of the acceleration of the component depend on maximum current

and masses of component and moving element.
 It generates force up to 30,000N and displacement 25mm.
 The operating-frequency range of the exciter lies between 5 to 20 kHz.

 Advantages
 It can have produced purely harmonic and constant force.
 It can have used wide frequency range.
 Easily measurement the excitation force.
 Disadvantages
o It has great mass and volume with respect to the amplitudes of the generating

exciting force.
C) ELECTRO-HYDRAULIC EXCITER
 Fluid pressure is used as power source of hydraulic vibration exciter.
 In this arrangement an electrically actuated servo valve operates a main control

valve, in turn regulating flow to each end of a main driving cylinder.
 Large capacities (up to 2 MN) and relatively high frequencies (to 400 Hz), with

amplitudes as great as 46 mm, have been attained.

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Measurement and Control of Vibration | THEORY

 COMPARISON OF EXCITER

D) IMPACT HAMMER
 Impact hammer is the one of the popular exciter.
 The equipment consists of a hammer, usually with a set of different heads and tips.
 Magnitude of the impact force detect by transducer.
 Manually excited the system by impact hammer.

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Measurement and Control of Vibration | THEORY

 ADVANTAGES & DISADVANTAGES

 Advantages:
o Impact hammer is simple, portable, inexpensive.
o It is much faster to use than a shaker.

 Disadvantages:
o It is often not capable of imparting sufficient energy to obtain adequate response

signals in the frequency range of interest.
o It is also difficult to control the direction of the applied force

1.9 VIBRATION BASED CONDITION MONITORING

 Condition monitoring of machine implies the determination of condition of a machine
and its change with respect to time.

 There are six steps to a healthy machine:
1. what are the possible failures?
1. which of these failures are significant?
2. how can we avoid these failures?
3. if we can’t avoid failure, can we get an early warning?
4. select a suite of tests to detect early warning signs.
5. collect the results of the tests at one decision point.

 Techniques that can be used include
o Human Senses
1. look,
2. listen,
3. smell,
4. taste,
5. feel
o Motor Current Analysis
o Oil Analysis and Tribology
o Non-destructive testing (NDT)

 VIBRATION MONITORING TECHIQUES
1) TIME DOMAIN
Acceleration Vs. Time
2) FREQUENCY DOMAIN
Acceleration Vs Frequency

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Measurement and Control of Vibration | THEORY

 MACHINE CONDITION CHART

2.0 STANDARDS RELATED TO VIBRATION MEASUREMENT
1 AS
2 BS
3 DIN
4 EN
5 GB (China)
6 IEC
7 ISO
8 JIS
9 MIL
10 VDI

o ANSI S2.13 - Mechanical Vibration of Non-Reciprocating Machines -
Measurements on Rotating Shafts and Evaluation

o BS 848-7 -Fans for general purposes. Specifications for balancing and vibration
(ISO 14694)

o DIN 50100 Testing of Materials; Continuous Vibration Test; Definitions, Symbols,
Procedure, Evaluation.

o EN 13059 Safety of industrial trucks - Test methods for measuring vibration
o ISO 11342 Mechanical vibration -- Methods and criteria for the mechanical

balancing of flexible rotors

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Measurement and Control of Vibration | THEORY

 B. CONTROL VIBRATION CONTROL

o Force Reduction of excitation inputs due to, for example, unbalance or
misalignment, will decrease the corresponding vibration response of the system.

o Mass Addition will reduce the effect (system response) of a constant excitation
force.

o Tuning (changing) the natural frequency of a system or component will reduce or
eliminate amplification due to resonance.

o Isolation rearranges the excitation forces to achieve some reduction or
cancellation.

o Damping is the conversion of mechanical energy (vibrations) into heat.

3.1 VIBRATION CONTROL IN MECHANICAL SYSTEMS
o The first level of methods attempts to reduce the excitation responsible for the
vibration at the source.
o The system parameters namely inertia, stiffness and damping are to be optimally
chosen or modified to reduce the response to a given excitation.
o The balancing of inertial forces, smoothening of fluid flows and proper lubrication
at joints are effective methods and should be considered whenever possible.
o Transmission of path of vibration needs to be worked on for effective isolation.
o Objective diagnostic tests and analysis can pinpoint the problem areas

 VIBRATION CONTROL METHODS
1. Excitation control of source
2. Source Isolation
3. System modification
4. Active feedback control

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Measurement and Control of Vibration | THEORY

1) EXCITATION CONTROL OF SOURCE
o Balancing of unbalanced inertia forces – rotors, engines, shafts
o Changing the flow characteristics for flow induced vibrations
o Proper lubrication of joints- Reducing friction, avoiding vortex shedding to reduce

self-excitation,
o Reduce parameter variation for parametric excitation
o Modification in surface finish
o Source provides the energy to maintain vibration. sources of vibration could be of

several types

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Measurement and Control of Vibration | THEORY

o Transient – for e.g. shock loading
o Forced excitation – Source (continuous) independent of Response
o Self-excited – Source generated by the Response for e.g. vortex induced vibration.
o Parametric excitation – System parameters (m, c or k) change with respect to time.

2) SOURCE ISOLATION
o Modify the transmission path of vibration between source and the system to

protect the system.
o Example - Insertion of resilient elements – Springs, Dampers, Viscoelastic

Materials, Pneumatic Suspension etc. between the source and the system.

3) SYSTEM MODIFICATION
o A large number of methods exist in this group including detuning, decoupling, using

additive damping treatments (constrained and unconstrained), stiffeners and
massive blocks (as foundation)
3.2 STEPS IN VIBRATION CONTROL
A. Identification and characterization of the source of vibration.
B. Specify the level to which the vibration should be reduced.
C. Select the method appropriate for realizing the vibration reduction level
identified in step B.
D. Prepare an analytical design based on the method chosen in step C.
E. Realize in practice (i.e. hardware mechanization of) the analytical design
constructed in step D.

3.3 VIBRATION ABSORBER
o In vibration analysis, a dynamic vibration absorber, or vibration neutralizer, is a tuned
spring-mass system which reduces or eliminates the vibration of a harmonically
excited system. Rotating machines such as engines, motors, and pumps often incite
vibration due to rotational imbalances.
o This reduces the possibility that a resonance condition will occur, which can cause
failure.
o Properly implemented, a dynamic absorber will neutralize the undesirable vibration

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Measurement and Control of Vibration | THEORY
 TYPES OF VIBRATION ABSORBERS
1. Undamped dynamic vibration absorber

(Suitable for constant speed machine)
2. Torsional vibration absorber
3. Centrifugal pendulum absorber

(Also known as Self tuned )
4. Untuned vibration absorbers

(Untuned dry friction damper)
(Untuned viscous damper)
1. UNDAMPED DYNAMIC VIBRATION ABSORBER

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Measurement and Control of Vibration | THEORY

3.4 VIBRATION ISOLATION
o The process of isolating the machine from the foundation is known as vibration
isolation.
o Objectives:
1. To protect the machine from excessive vibration transmitted to it from its
supporting structure.
2. To prevent vibratory forces generated by machine from being transmitted to its
supporting structure.
o The effectiveness of isolation may be measured in terms of force or motion
transmitted to that existence.
o Accordingly, it’s known as force isolation or motion isolation. The lesser the force
or motion transmitted the grater is the isolation

 METHODS OF VIBRATION ISOLATION
o PASSIVE VIBRATION ISOLATION

Refers to vibration isolation or mitigation of vibrations by passive techniques such
as rubber pads or mechanical springs. (Without external power source)
o ACTIVE VIBRATION ISOLATION
Also known as electronic force cancellation Employs electric power, sensors,
actuators, and control systems for vibration isolation. (With external power source)
o SEMI ACTIVE VIIBRATION ISOLATION
(Electro-Rheological & Magneto-Rheological fluid based damper)
ER fluids which changes their viscosity by application of electric fluid.

 TYPES OF PASSIVE VIBRATION ISOLATORS
1) Springs
o Steel coil springs are used
o Natural frequency down to about 8Hz.

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Measurement and Control of Vibration | THEORY
2) Rubbers or Elastomers
o Natural rubber, neoprene isolators.
o Used in compression and shear.
o For static deflection of 10 to12mm
o Natural frequency down to about 5Hz.

3) Ribbed elastomers
o For static deflection of 10 to12mm
o Natural frequency down to about 10Hz.

4) Pneumatic isolator
o Natural frequency down to about 10Hz.
o Pneumatic isolators or air springs or air mount are used
o Lateral stability, internal damping

5) 5. Other materials
o Wool, felt, foam, glass fibers

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Measurement and Control of Vibration | THEORY

 ACTIVE VIBRATION ISOLATION
o Feedback systems
o Works by applying equal and opposite forces
o Requires external power source
o Cost is higher
o Sensitive instruments

 IMPORTANT CONSIDERATIONS WITH VIBRATION ISOLATOR SELECTION
1) Machine Location:

As far away from sensitive areas as possible on as rigid a foundation as possible
2) Proper sizing of isolator units:

Correct stiffness:
Sufficient travel to prevent bottoming out during shock loads, or during system
startup and shutdown
3) Location of isolators:
Isolators should be equally loaded, and the machine should be level
4) Stability:
Sideways motion should be controlled
The diameter of the spring should also be greater than its compressed height
5) Adjustment:
Springs should have free travel, should not be fully compressed, nor hitting a
mechanical stop
6) Eliminate vibration short circuits:
Any mechanical connection between machine and foundation which bypasses the
isolators, such as pipes, conduits, binding springs, poorly adjusted rubbers or
mechanical stops
7) Safe operation:
To avoid a spring break, you must have mechanical supports on which the machine
can rest without tipping

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